In inter-vehicle networks, it is required that collision warning messages be distributed over multiple hops with significantly short delay. Cut-through packet forwarding is known to be an effective method to reduce forwarding latency. However, it suffers from the reliability issue, which is especially severe in highly mobile scenario, due to multi-path fading. This paper tries to establish a reliable cut-through forwarding scheme by applying diversity combining method to collision warning messages. By exploiting the nature of the multi-hop forwarding of a broadcast packet in CDMA wireless networks, the diversity combining scheme does not incur any additional communication overhead or delay. The simulation results show that diversity combining schemes, especially Maximal Ratio Combining (MRC), can effectively reduce BER by orders of magnitude.

Wireless mesh networks are attracting more and more attention as a promising technology for the next generation access infrastructure. QoS support is a unavoidable task given the rising popularity of multimedia applications, and also a challenging task for multi-hop wireless mesh networks. Among the numerous QoS factors, end-to-end delay is one of the most critical and important issues, especially for the real time applications. Over multi-hop wireless mesh networks, end-to-end delay of a flow is highly dependant on the number of hops as well as congestion condition of the hop nodes that the flow traverses through. In this paper, we propose QoS priority control schemes based on the end-to-end QoS delay metrics in order to increase traffic accommodation, i.e., the numbers of real-time flows which satisfy the requirements of end-to-end delay and packet delivery ratio over multi-hop wireless mesh networks. The first scheme enables source and forwarding nodes to perform priority control based on the number of hops of routes. The second scheme enables nodes to perform priority control based on the congestion condition of the hop nodes, where the flow traverses through. The effectiveness of the proposed schemes is investigated with NS-2 network simulator for voice and video traffics over multi-hop wireless mesh networks. Simulation results show that the scheme greatly improves the traffic accommodation for voice and video applications in multi-hop wireless mesh networks.

In Mobile Ad hoc Networks (MANET) geographic routing is characterized by local forwarding decision. Links with a long progress are preferred under the greedy forwarding rule. However in a real system long links tend to have a high packet loss rate due to multipath fading. A sub-optimal solution may separately exploit path diversity or rate adaptation. In this paper we study channel efficiency of multi-hop forwarding and try to jointly optimize rate adaptation and forwarder selection in geographic routing by the tradeoff between progress and instantaneous rate. We define a new metric -- Bit Transfer Speed (BTS) -- as the ratio of the progress made towards the destination to the equivalent time taken to transfer a payload bit. This metric takes overhead, rate and progress into account. Then we propose a packet forwarding scheme that Opportunistically exploits both long Progress and Adaptive Rate (OPAR) by a cross-layer design of routing and MAC. In OPAR each node selects for a packet the forwarder with the highest BTS. The forwarder changes as local topology (progress), packet size (overhead ratio) or channel state (data rate) varies. Simulation results show that compared with the normalized advance (NADV) [7] scheme and contention-based forwarding (CBF) [17] scheme, OPAR has lower packet loss and can effectively reduce channel occupation time by over 30% in the scenario with moderate mobility speeds.

IEEE 802.11 DCF is an asynchronous and distributed MAC protocol which does not require the existence of a central controller for medium access coordination. This flexibility, which is due to DCF's contention-based nature, comes at the expense of the overhead associated with contention resolution. The overhead consists of frame collision time and channel idle time, which is particularly severe when channel is saturated. In this paper, we present an enhancement of DCF which aims at reducing its contention resolution overhead by equipping it with a distributed reservation mechanism. The proposed reservation mechanism enhances collision avoidance mechanism of DCF by enforcing a partially ordered medium access through an implicit agreement between neighboring nodes. Simulation results, using ns-2 network simulator, show that the added reservation scheme 1) effectively reduces DCF's overhead and improves channel utilization particularly when node density and traffic load is high, 2) significantly enhance DCF's fairness.